The invention relates to a circuit device for feeding a discharge lamp, comprising
input terminals for connecting the circuit device to a supply-voltage source,
a ballast circuit for generating a lamp current from a supply voltage supplied by the supply-voltage source,
a circuit part I, which is coupled to the ballast circuit, for controlling the power consumed by the discharge lamp in dependence upon a dim signal,
a circuit part II, which is coupled to the circuit part I, for generating the dim signal, which dim signal has a first value immediately after the ignition, which value corresponds, during stationary operation, to a first value of the power consumed by the discharge lamp, and is maintained at a second value after a predetermined time interval, which second value corresponds, during stationary lamp operation, to a second value of the power consumed by the discharge lamp, which second value is lower than the first value of the power consumed by the discharge lamp,
a circuit part V which enables a user to set a dim signal to the second value or a third value which, during stationary lamp operation, corresponds to a third value of the power consumed by the discharge lamp, which third value is higher than the second value of the power consumed by the discharge lamp.
Such a circuit arrangement is disclosed in WO 94/25912. In the known circuit device, the first value of the dim signal corresponds to rated power. During the predetermined time interval, this first value does not change. Both the circuit arrangement and the discharge lamp fed by means of the circuit arrangement heat up during the predetermined time interval to a temperature at which stable lamp operation in a dimmed state is possible. After the predetermined time interval, the circuit part II sets the value of the dim signal to a second value which corresponds to a second value of the power consumed by the discharge lamp, at which second value the discharge lamp bums in a dimmed state. An important advantage of the known circuit device resides in that the luminous flux of the lamp adjusts to a dimmed level without the interference of a user, so that the power consumed by the discharge lamp is relatively low. If the luminous flux at the dimmed level is found satisfactory by the user, a substantial energy saving is obtained in this manner. In practice, it has been found that if the discharge lamp connected to the circuit device is comparatively new, the luminous flux obtained when the discharge lamp burns at a dimmed level is found satisfactory by many users. As a result of ageing of the discharge lamp, however, the luminous flux at a given power is subject to deterioration. If the luminous flux at the dimmed level is found to be insufficient by the user, he can activate the circuit part V by xe2x80x9ctogglingxe2x80x9d the main switch which connects the circuit device to the supply voltage source. As a result, said circuit part V then adjusts the dim signal to a third value, which corresponds, in the known circuit device, to the rated lamp power and to the maximum luminous flux of the discharge lamp.
A drawback of the known circuit device, however, resides in that the luminous flux of the discharge lamp fed by means of the circuit device changes continuously during the predetermined time interval. This can be attributed to the fact that the temperature of the discharge lamp changes continuously during the predetermined time interval. In addition, the luminous flux changes abruptly towards the end of the predetermined time interval, which can be attributed to the fact that the circuit part II abruptly sets the dim signal to the second value. These comparatively large changes of the luminous flux within a comparatively short period of time are generally perceived as disturbing by a user.
It is an object of the invention to provide a circuit device which enables a substantial saving in energy to be obtained without the luminous flux of the discharge lamp fed by the circuit arrangement being subject to large changes within a comparatively short period of time.
To achieve this, a circuit device of the type mentioned in the opening paragraph is characterized in accordance with the invention in that, during the predetermined time interval, the dim signal is adjusted by the circuit part II in such a manner that, during the predetermined time interval, the luminous flux of the discharge lamp differs less than 25% from the luminous flux which, during stationary operation, corresponds to the second power consumed by the discharge lamp.
By virtue of the fact that the circuit part II continuously adapts the dim signal during the predetermined time interval, it is achieved that the luminous flux varies only to a very small degree. The level at which the luminous flux is maintained during the predetermined time interval is chosen to be equal to the luminous flux level which, under stationary conditions, corresponds to the second value of the dim signal, in this case the second value of the power consumed by the discharge lamp. As a result, it is achieved that the luminous flux is not subject to substantial change at the end of the predetermined time interval, but remains substantially constant instead. From the moment the discharge lamp is ignited and during the time the discharge lamp bums, the user of a discharge lamp in accordance with the invention continually observes a substantially constant quantity of light. It has been found that this is appreciated by users.
It has also been found that, preferably during the predetermined time interval, the dim signal is adapted by the circuit part II in such a manner that the luminous flux of the discharge lamp differs less than 10% during the predetermined time interval from the luminous flux which, during stationary operation, corresponds to the second power consumed by the discharge lamp.
The circuit part II of a circuit device in accordance with the invention may be provided with analog electronics to implement the desired time dependence of the dim signal during the predetermined time interval. In this manner, the circuit device can be made suitable for at least a first type of discharge lamp. If such a circuit device would be used in combination with a discharge lamp of a second type whose run-up behavior differs from the run-up behavior of the discharge lamp of the first type, it is possible that the luminous flux of the discharge lamp of the second type changes substantially during the predetermined time interval. For this reason, the circuit part II is provided, in a first embodiment of a circuit device in accordance with the invention, with
a memory for storing dim-signal values for a number of instants in the predetermined time interval,
a circuit part III for successively making the dim signal equal to the values stored in the memory.
The contents of the memory can be attuned to the properties of the type of discharge lamp fed by means of the circuit device. The circuit device can thus be made suitable for use in combination with a large number of different types of discharge lamps by adapting the contents of the memory. Good results have been achieved for embodiments of such a first preferred embodiment wherein the circuit part III comprises a microprocessor.
Good results have also been attained for embodiments of a circuit device in accordance with the invention, wherein the circuit part II is provided with a circuit part VI for controlling the luminous flux. This circuit part VI is preferably provided with
a light sensor for generating a first signal which is a measure of the luminous flux of the discharge lamp,
a comparator provided with a first and a second input, the first input being coupled to the light sensor,
a circuit part IV coupled to the second input of the comparator for generating a reference signal which is a measure of the value of the luminous flux which, during stationary operation, corresponds to the second power consumed by the discharge lamp, and
a control circuit coupled to an output of the comparator for generating a dim signal in dependence upon the first signal and the reference signal.
Via the comparator, the control circuit continuously adjusts the dim signal to such a value, during the predetermined time interval, that the luminous flux of the discharge lamp remains substantially constant. Also for these embodiments, it applies that they can be used in combination with discharge lamps having a different run-up behavior.
Preferably, the circuit part V is embodied so as to be activatable by means of interrupting the supply voltage. In this case, the dim signal can be given the second or the third value by a user by interrupting the supply voltage for a short period of time by means of the main switch which connects the supply voltage source to the circuit device.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.